High interrupting capacity puses having	casings
of inorganic material



Dec. 13, 1966 F. J. KOZACKA ETAL 3,291,940

HIGH INTERRUPTING CAPACITY FUSES HAVING CASINGS OF INORGANIC MATERIALFiled June 19, 1964 2 Sheets-Sheet l INVENTORS= FREDERICK J. KOZACKA,

BY WWW TTORNEY 13, 1966 F. J. KOZACKA ETAL 3,291,940

HIGH INTERRUPTING CAPACITY FUSES HAVING CASlNGS OF INORGANIC MATERIALFlled June 19, 1964 2 Sheets-Sheet 2..

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BY fimmgf TORNEY HIGH INTERRUPTING CAPACITY FUSES HAVING CASINGS FINORGANIC MATERIAL Frederick J. Kozacka, South Hampton, N.H., and ErwinSalzer, Waban, Mass., assignors to The Chase-Shawmut Company,Newburyport, Mass.

Filed June 19, 1964, Ser. No. 376,417 3 Claims. (Cl. 200-120) Thisapplication is a continuation-in-part of our c0- pending patentapplication Ser. No. 355,804, filed March 20, 1964 for ElectricalCartridge Fuses, and this invention is concerned with high interruptingcapacity fuses.

It is one object of this invention to provide high interrupting capacityfuses having casings of an inorganic insulating material which fuses arenot subject to the drawbacks and limitations of prior art fuses havingcasings of inorganic materials.

The most common inorganic insulating materials for making casings ofelectric fuses are glass, porcelain and steatite. The least desirable ofthese materials is glass because of its brittleness. A seriouslimitation of porcelain and steatite as casing materials for fusesresides in the fact that these materials require a firing operationduring which operation they shrink, making it impossible to maintain theclose tolerances which are necessary, or desirable, for fuse casings offuse tubes. After the firing operation ceramic materials are veryhard-steatite is then harder than glass-and for this reason it ispossible to machine pieces of fired ceramics only by grinding withCarborundum or diamond wheels.

It is, therefore, another object of this invention to provide highinterrupting capacity fuses having casings of inorganic insulatingmaterials not subject to the aforementioned limitations of casings ofglass, porcelain and steatite.

Under short-circuit current conditions fusion of fusible elements offuses and arcing is a process in the nature of an explosion. Thereforethe dynamic strength of a fuse casing is a matter of paramount practicalimportance. Dynamic strength requirements of a fuse structure having arequired current carrying capacity can best be met by integrating aplurality of separate tubular casing structures each housing a separatefusible element and a separate body of arc-quenching filler into astructural unit. Such multiple fuse structures may be achieved bysubstituting for the conventional casing of a fuse a cylindrical blockof a ceramic material having a plurality of axially extending bores ofwhich each houses one of a plurality of fuse links connected in parallelinto an electric circuit and of which each also houses a separate bodyof pulverulent arc-quenching tiller. While multiple fuses of thischaracter have a relatively high dynamic strength, they are subject toall the limitations and drawbacks of fuses having a casing of a ceramicmaterial. These drawbacks can be avoided and the high dynamic strengthinherent in the multiple unit design still achieved by integrating aplurality of conventional fuse structures each having a relatively smallcurrent carrying capacity by means of common terminal blocks into aunitary fuse structure having a relatively high current carryingcapacity. Fuses of this character are disclosed and claimed in US.Patent 2,647,970 to Wm. S. Edsall et al., Aug. 4 1953 Current-LirnitingFusible Protective Device and US. Patent 2,653,203 to F. I. K-ozacka,Sept. 22, 1953 Current Limiting Fuse, both assigned to the same assigneeas the present invention. Multiple fuses of the kind disclosed in thetwo above referred-to patents are, however, relatively complex anddifiicult to manufacture and do not lend themselves to use in connectionwith inorganic casing materials.

It is, therefore, another object of this invention to pro- United StatesPatent 0 3,291,940 Patented Dec. 13, 1966 vide multiple fuse structureswithin the above meaning which are not subject to the limitations anddrawbacks of the above referred-to prior art multiple fuse structures.

If the spring biased contacts of a fuse holder are intended to beardirectly against the end surfaces of a fuse structure the casing of thelatter must be kept to relatively close tolerances, and must have a highdegree of dimensional stability, and a relatively high compressivestrength, for lack of any of these characteristics results in anundesirable, or intolerable, variation of the contact pressure frompiece to piece of equipment. Combinations of fuse holders and fuses ofthe aforementioned kind are, therefore, ruled out wherever the casing ofthe fuse is made of a fired ceramic material, for the tolerances of suchmaterials are too large to achieve the required uniformity of contactpressure. Combinations of fuse holders and fuses of the aforementionedkind are also ruled out wherever the casings of the fuses consist offiber; for fiber is a material whose dimensional stability is far toosmall to achieve the required uniformity of contact pressure.

It is, therefore, another object of this invention to provide electricfuses having casings of a relatively inexpensive inorganic insulatingmaterial which has the physical characteristics required to allow thecontacts of a fuse holder to engage under pressure the end surfaces ofthe fuse structure.

The present invention as well as the aforementioned patent applicationSer. No. 355,804 are concerned with fuses having casings ofprecompressed asbestos cement. The term precompressed asbestos cement asused in this context is intended to distinguish a specific kind ofasbestos cement from other kinds of asbestos cement. The termprecompressed asbestos cement is applied to identify a laminated,densified product obtained by bonding thin, soft, paperlike laminae(which may have been made on a paper making machine) under considerablepressuresuch as 1,400 p.s.i.into a unitary material, simultaneouslyremoving a great deal of excess water. The process of makingprecompressed asbestos cement, or densified asbestos cement (which isknown in the art) will be outlined below more in detail. Casings ofprecompressed asbestos cement can readily be machined. This makes itpossible to manufacture fuse casings out of relatively thick sheets ofprecompressed asbestos cement by simple cutting or sawing and drillingoperations. The geometry of such casings tends to increase dynamicstrength if they are formed by prismatic blocks having a plurality ofparallel bores each receiving one of a plurality of fuse links and apulverulent arc-quenching filler. This, in turn, is conducive to greatcompactness for given rating requirements. The relatively greatcompressive strength of precompressed asbestos fiber cement coupled withthe close tolerances to which that material can readily be machined makeit possible to associate fuses having casings of precompressed asbestoscement with fuse holders having contacts directly engaging the endsurfaces on the fuses and putting the casings of the latter undercompression. As mentioned above, fuse casings of fired ceramicmaterials, while having a high compressive strength, do not lendthemselves to application in such fuse holders because such fuse holderscan be applied only if the spacing between the end surfaces of the fusescan be held to close tolerances, and because fired ceramic materials donot allow to achieve sufiiciently close tolerances.

Since fuse holders having contacts which engage directly the endsurfaces of a fuse structure make it possible to dispense with bladecontacts on the latter, and to arrange the fuse holder contacts closerto the fuse structure, combinations of this nature of fuse holders andfuses having casings of organic insulating materials.

3 are conducive to both greater economy and greater compactness.

Providing a plurality of fuse-link-receiving bores in prismatic blocksof precompressed asbestos cement-the axes of the bores being arranged inplanes parallel to two of the outer surfaces of the blocksyields fusestructures which are relatively narrow and relatively deep. This is verydesirable for a number of applications where there are no stringentlimitations in regard to depth requirements, but stringent limitationsin regard to width requirements. One such instance is the combination offuses with automatic switches and/or circuit breakers, where fuses mayhave a depth commensurate to that of the automatic switch or circuitbreaker, but should be relatively flat to achieve large spacings betweenthe phases, or the poles, of an automatic switch or circuit breaker.

Fuses and combinations of fuses and fuse holders embodying our inventionare illustrated in the accompanying drawings wherein:

FIG. 1 is a side elevation of a fuse embodying this invention;

FIG. 2 is a top plan view of the structure of FIG. 1;

FIG. 3 is a side elevation of another fuse embodying this invention;

FIG. 4 is a top plan view of the structure of FIG. 3;

FIG. 5 is a vertical section across another fuse embodying thisinvention taken along 55 of FIG. 6; and

FIG. 6 is a top-plan View of the structure of FIG. 5.

Referring now to the drawings, and more particularly to FIGS. 1 and 2thereof, numeral 26 has been applied to indicate a prismatic block ofprecompressed asbestos cement. Block 1 is rectangular in cross-sectionand has a pair of rectangular end surfaces 26'. Block 26 further definesa plurality of bores 25 circular in cross-section and extending at rightangles to end surfaces 26'. Fusible ribbon elements 24 of a highcurrent-limiting action metalcopper or silver-is arranged within bore 25in coaxial relation thereto. Fusible ribbon elements 24 of a highcurrent-limiting action metalcopper or silveris arranged within bore 25in coaxial relation thereto. Fusible ribbon elements 24 are providedwith a plurality of serially related points of reduced cross-section onnecks (not shown in FIGS. 1 and 2). Bones 25 are filled with a body ofquartz sand, as clearly shown in FIGS. 3 and 5. As a result of the useof quartz sand as arc quenching medium, and of the compactness of thedevice, block 26 may reach, or will reach, temperatures not permissiblein fuses Since block 1 is made of precompressed asbestos cement it cansafely withstand temperatures far in excess of those to which organiccasing materials may be subjected. Precompressed asbestos cement iscapable of safely withstanding temperatures of 600 F. for indefiniteperiods of time. The end surfaces 26' of block 26 are covered bycongruent stampings 27', 28' of asbestos sheet and asbestos sheetstampings 27', 28' are, in turn, covered by terminal plates 27, 28 ofmetal. The latter have an angular flange which is intended to affix thefuse structure to a fuse holder. Asbestos stampings 27', 28' areprovided with central slits for the passage of fuse links 24. Each ofthe lower ends of fuse links 24 may have a bend of 90 (not shown inFIGS. 1 and 2) spot-welded to the lower terminal plate 28. The upperterminal plate 27 has a plurality of circular depressions or recesses 22intended to receive a small quantity of solder. The upper end of eachfuse link 24 is threaded through a narrow hole in terminal plate 27 andsoldered to the latter by a small quantity of solder neatly limited tothe area of recesses or depressions 22. Terminal plates 27 and 28 areaflixed to block 1 by means of self-tapping drive pins 29 projectingthrough terminal plates 27, 28 into end surfaces 26' of block 26. Tothis end block 26 is provided on the upper side thereof and on the lowerside thereof with eight axial holes at the places where drive pins 29are intended to be located. These holes are not tapped, and the drivepins are driven into them by means of a suitable press. While subjectedto the pressure of the press drive pins 29 rotate about their own axes,and while so rotating cut appropriate threads into block 26.

It will be apparent that this simple technique of affixing terminalplates cannot be applied in instances where the casing structure is madeof a fired ceramic material since such materials are too hard to allowthe use of selftapping pins.

It will be apparent from FIGS. 1 and 2 that there are six fuse link andquartz-sand-receiving bores 25 in block 26 of precompressed asbestoscement. These six bores 25 in the block 26 shown in FIGS. 1 and 2 arearranged in two groups, each comprising three bores 25. There is onedrive pin 29 in each corner of terminal plates 27, 28, and there areadditional pairs of drive pins between contiguous pairs of bores 25.This even spread of fasteners 29 is of considerable importance since itfully meets the increased dynamic strength requirements of the structureof FIGS. 1 and 2 and does not call for drive pins of relatively largediameter tending to impair the mechanical integrity of block 26.

The structure of FIGS. 3 and 4 will be described below only to theextent that it differs significantly from the structures previouslydescribed. The prismatic block 30 of precompressed asbestos cement shownin FIGS. 3 and 4 defines three bores 31, 32, 33 having axes arranged ina common plane parallel to two of the outer surfaces of block 30. Bores31, 32 house arrangements of fuse links and of quartz sand such as morefully described in connection with FIGS. 1 and 2. Bore 33 houses a blownfuse indicator mechanism, or striker pin mechanism, for indicatingwhether or not the fuse has blown, or for performing some mechanicalduty such as, for instance, tripping of a circuit breaker, or load breakdisconnect, in response to blowing of the fuse. The structure housed inbore 33 of FIGS. 3 and 4 includes a restraining wire 34 of steelthreaded through lower asbestos fiber stamping 35 and lower terminalplate 36 of metal and having one end clamped by screw 37 against plate36. The upper end of restraining wire 34 is attached to the lowertabshaped and perforated end of an indicator or striker pin 38. Thelatter is surrounded by a helical spring 39 biasing pin 38 and springhousing 40 in upward direction. The lower end of helical biasing springrests against potshaped bore insert 41 having a perforation 42 for thepassage of striker pin 38 and closing the upper end of bore 33.Restraining wire 34 is threaded at the lower end thereof through alittle disc 43 of an elastomer forming a seal precluding the loss of anyof the quartz sand filler 44 inside of bore 33.

Parts 41, 40, 38, 34 and 37 establish a conductive connection betweenthe upper terminal plate 45 and the lower terminal plate 36. Thiscurrent path has a high ohmic resistance and is shunted by the fuselinks 24 in bores 31 and 32 establishing current paths of relativelysmall ohmic resistance between terminal plates 36 and 45. Therefore thecurrent path formed by parts 41, 40, 38, 34 and 37 does not normallycarry any significant cur-rent, virtually all the current being carriedby the fuse links 24 in bores 31 and 32. Upon blowing of the fuse thesefuse links 24 are severed and a significant current begins to flowthrough restraining wire 39, resulting in rapid fusion thereof. Nowspring 39 is free to propel pin 38 and spring housing 40 out of bore 33in block 30, indicating that the fuse has blown, or performing someauxiliary operation in response to blowing of the fuse.

The structure of FIGS. 5 and 6 comprises a prismatic block46 ofprecompressed asbestos cement having two parallel bores 47, 48, eachhousing a fuse link 49, 50 and a body of quartz sand 51, 52. Terminalplates 53, 54 are secured to block 46 by self-tapping drive pins 55, andconductively interconnected by fuse links 49, 50.

Each terminal plate 53, 54 is formed by two flange portions of which oneis horizontal and the other vertical. Each vertical flange portionsupports a cylindrical rod or pin contact 56, 57 cooperatively engaginga tulip contact generally indicated at 58 and 59, respectively.

Regarding the product which has been referred-to above as precompressed,asbestos cement, the preparation of that product differs from and itspreparation dilfffer from the preparation, of other asbestos cementproducts. One of these differences resides in the fact that in makingprecompressed asbestos cement large quantities of excess water are beingused, which are subsequently removed under considerable pressure inhydraulic presses before the asbestos cement is allowed to cure. Hencethe term precompressed asbestos cement. The proportions of theingredients, i.e. asbestos and hydraulic cement, may vary within broadlimits. Thus a thin slurry may be prepared including 8 to 20% ofPortland cement, about of asbestos, and water. For this particualrapplication it is desirable to use asbestos known as Chrysotil. Thecement slurry may be processed on a paper making machine, resulting inthe removal of some excess water through sieves. Further removal ofexcess water may be achieved by supporting the asbestos cement mixtureon a support of felt. The sheets delivered by the paper making machineare cut to the desired size and stacked. Then a solid metal plate isarranged on the top of each stack and a Wire net on the bottom of eachstack. A number of such stacks are superimposed in a heavy hydraulicpress and subjected to considerable pressure resulting in a furtherremoval of excess water prior to curing of the stacks. The uniformplates of precompressed asbestos cement resulting from this process arecut to strips, whereupon the latter are cut to prismatic blocks andprovided by means of gang operated drills with the required number oflink-receiving bores.

It will be understood that we have illustrated and described hereinpreferred embodiments of our invention and that many alterations may bemade in the details thereof without departing from the spirit and scopeof our invention as defined in the appended claims.

We claim:

1. An electric fuse comprising in combination:

(a) a plurality of fusible elements of a high currentlimiting actionmetal;

(b) a plurality of separate bodies of quartz sand each embedding one ofsaid plurality of fusible elements;

(c) a pair of spaced terminal plates conductively connected by saidplurality of fusible elements; and

(d) a prismatic block of precompressed asbestos cement having a pair ofend surfaces and defining a plurality of bores extending at right anglesto said pair of end surfaces, the axes of said plurality of bores beingsituated in a common plane parallel to a pair of ourter surfaces of saidblock, each of said plurality of fusible elements and each of saidplurality of bodies of quartz sand being housed in one of said pluralityof bores, said pair of terminal plates covering said pair of endsurfaces of said block; and (e) self-tapping drive pins projectingthrough said pair of terminal plates into said pair of end surfaces ofsaid block for securing said pair of terminal plates to said block.

2. An electric fuse comprising in combination:

(a) a plurality of fusible elements of a high currentlimiting actionmetal;

(b) a plurality of separate bodies of quartz sand each embedding one ofsaid plurality of fusible elements;

(c) a pair of spaced terminal plates conductively connected by saidplurality of fusible elements;

(d) a prismatic block of precompressed asbestos cement having a pair ofrectangular end surfaces and defining a plurality of bores extending atright angles to said pair of end surfaces, the axes of said plurality ofbores being arranged in a common plane parallel to a pair of outersurfaces of said block, each of said plurality of fusible elements andeach of said plurality of bodies of quartz sand being housed in one ofsaid plurality of bores; and

(e) a plurality of pairs of self-tapping drive pins projecting througheach of said pair of terminal plates into each of said pair of endsurfaces of said block, said plurality of pairs of self-tapping drivepins including pairs of self-tapping drive pins arranged betweencontiguous ends of said plurality of bores.

3. An electric fuse comprising in combination:

(a) a predetermined number of fusible ribbon elements of a highcurrent-limiting action metal;

(b) a pair of spaced terminal plates conductively connected by saidribbon elements;

(0) a prismatic block of precompressed asbestos cement having a pair ofrectangular end surfaces and defining a plurality of bores extending atright angles to said pair of end surfaces and exceeding by one saidpredetermined number of fusible ribbon elements, said plurality of boreshaving a common median plane parallel to one pair of outer surfaces ofsaid block, each of said fusible ribbon elements being housed in one ofsaid plurality of bores and embedded in a body of quartz sand, said pairof terminal plates covering said pair of end surfaces of, said block;

(d) a blown fuse indicator in one of said plurality of bores, said oneof said plurality of bores further housing a fusible wire elementconductively connecting said pair of terminal plates and normallyrestrining said blown fuse indicator, and a separate body of quartz sandin said one of said plurality of bores embedding said fusible wireelement; and

(e) self-tapping pins projecting through said pair of terminal platesinto said pair of end surfaces of said block for securing said pair ofterminal plates to said block.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCESIohns-Manville (I), Transite, publication No. IN-184 A, June 1959, pp.2-4.

Iohns-Manville (II), New Asbestos Ebony and Ohmstone, publication No.IN-229A, December 1959, pp. 2 and 3.

R. T. Lythall: The Switchgear Book, Wrayburg,

England, 1953, pp. 215-217. TK 2921. L916.

BERNARD A. GILHEANY, Primary Examiner. H. B. GILSON, Assistant Examiner.

1. AN ELECTRIC FUSE COMPRISING IN COMBINATION: (A) A PLURALITY OF FUSIBLE ELEMENTS OF A HIGH CURRENTLIMITING ACTION METAL; (B) A PLURALITY OF SEPARARE BODIES OF QUARTZ SAND EACH EMBEDDING ONE OF SAID PLURALITY OF FUSIBLE ELEMENTS; (C) A PAIR OF SPACED TERMINAL PLATES CONDUCTIVE CONNECTED BY SAID PLURALITY OF FUSIBLE ELEMENTS; AND (D) A PRISMATIC BLOCK OF PRECOMPRESSED ASBESTOS CEMENT HAVING A PAIR OF END SURFACES AND DEFINING A PLURALITY OF BORES EXTENDING AT RIGHT ANGLES TO SAID PAIR OF END SURFACES, THE AXES OF SAID PLURALIITY OF BORES BEING SITUATED IN A COMMON PLANE PARALLEL TO A PAIR OF OUTER SURFACES OF SAID BLOCK, EACH OF SAID PLURALITY OF FUSIBLE ELEMENTS AND EACH OF SAID PLURALITY OF BODIES OF QUARTZ SAND BEING HOUSED IN ONE OF SAID PLURALITY OF BORES, SAID PAIR OF TERMINAL PLATES COVERING SAID PAIR OF END SURFACES OF SAID BLOCK; AND (E) SELF-TAPPING DRIVE PINS PROJECTING THROUGH SAID PAIR OF TERMINAL PLATES INTO SAID PAIR OF END SURFACES OF SAID BLOCK FOR SECURING SAID PAIR OF TERMINAL PLATES TO SAID BLOCK. 